To the OP:
Only take it if you consider doing something that requires or is related to what you want to do at uni. Don't take it just for a filler, you'll probably end up thinking I can't be arse with it and might not do that great.
Once you understand the basics, you can do any of the other stuff, that's what I found anyway.
I did both Phyiscs and Chemistry at A-level, i'd say Physics was slightly harder. I didn't do Biology but from what I've heard Biology is the easiest out of the three. So I'd say it's 1) Physics , 2) Chemistry and 3) Biology